A known aesthetic body shaping technique is the use of ultrasound to treat fat tissue layers below the skin. A problem with known systems is that of directing or concentrating the ultrasonic energy to only affect or treat the desired volume of tissue, i.e. the fat cells, without affecting other regions of the skin. A related problem is that, for ultrasonic transducers, the power transmitted falls away with distance from the transducer. This can be addressed using high-intensity focussed ultrasound, but this is a relatively complex solution. An example of an alternative device is known from US2012/0277587 showing a treatment device which has a cavity into which skin surface tissue is drawn by applying a vacuum. The treatment device comprises two parallel ultrasonic transducers which direct energy into the tissue between them. To provide an effective treatment volume between a pair of transducers however, the power has to be raised sufficiently such that surrounding tissue may be affected and damaged. Circular transducers are also known to provide a strong focal point for the ultrasound but the focal point is relatively small and the transducers are expensive.